On the left, and I pretty much always sleep on my left side. Just replaced the old double duvet for a king-sized, which has reduced the tendency for me to be left with the very edge of it; oddly I always get accused of hogging the duvet that herself has wrapped herself in!
I only have one pillow, while herself has two, ‘cos she often sleeps on her stomach cuddling the thicker top pillow.
I honestly don’t understand how she can sleep like that!